Field
The disclosed concept relates generally to ground fault circuit interrupters (GFCI), and more particularly, to self-testing GFCIs. The disclosed concept also relates to methods of testing GFCIs.
Background Information
Electrical circuits generally employ one or more circuit interrupters configured to disable power to a load in response to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. One such circuit interrupter is a ground fault circuit interrupter (GFCI). A GFCI is a device that disables an electric circuit when it detects that current is flowing along an unintended path, such as through water or through a person. GFCIs are often used to reduce the risk of electric shock. GFCIs are available in two types for permanent installation, the circuit breaker type that may be installed in a circuit breaker panel, and the receptacle type that may be installed in a normal electrical box.
A GFCI works by measuring difference between the current leaving the hot side of the power source and the current returning to the neutral side. If the measured currents are not equal (thus making the difference zero), this means that some of the current is flowing along an unintended path, and the GFCI shuts the power off. When the problem is corrected, the GFCI can manually be reset by pushing a reset button provided as part of the GFCI.
GFCIs are covered by Underwriters Laboratory (UL) Standard UL 943. The Standard UL 943 requires that GFCI devices include a built-in test circuit including a test button which allows a user to periodically manually test the device. When the test button is depressed, a simulated ground fault current is produced that causes the GFCI device to open if the device is operating properly. The device must then be manually reset (for receptacle types, this is done by pressing the rest button to return it to service; for circuit breaker types, this is done by switching the manual handle back to ON).
More recently, GFCIs have been developed that employ a self-test functionality that automatically performs a self-testing process periodically (e.g., without limitation, every 3 hours, every hour, or every minute). Such self-test GFCIs of the receptacle type typically include a status indicator, such as one or more LEDs, to visually indicate the current status of the device (i.e., operational or not operational) so that it can be replaced in the case of a failure. For breaker type GFCIs, the breaker will de-energize (trip) circuit if a self-test does not pass.
The self-testing process in GFCIs is more extensive than the simulated ground fault current that is produced when the user presses the test button. However, the user cannot manually initiate the self-testing procedure and can only assume that the self-testing process is being performed periodically. A fault in the timing logic of a GFCI could cause the GFCI to fail to periodically perform the self-testing process and the user would have no way to trigger the self-test process.
There is room for improvement in GFCIs, and in methods of testing GFCIs.